This invention relates to an improvement in making polyethylene terephthalate (PET) useful in fiber, film, and molding applications.
PET may be prepared, as is well known, by the ester interchange of dimethyl terephthalate with ethylene glycol or by the direct esterification of ethylene glycol and terephthalic acid, followed by condensation polymerization (hereinafter "polycondensation") in the presence of a catalyst such as antimony trioxide (Sb.sub.2 O.sub.3), e.g., at a temperature of about 275.degree. to 300.degree. C. The PET product may then be extruded and pelletized to produce polymer chip.
Depending on the particular end use intended for the polymer, the polymer chip may then be subjected to a solid state polymerization process in order to increase the polymer's physical properties, i.e. to increase intrinsic viscosity and/or to remove acetaldehyde trapped in the pellets during manufacture. It is widely known in that art that the intrinsic viscosity of PET may be increased by solid state polymerization. In this connection, see e.g. U.S. Pat. Nos. 4,223,128 and 4,064,112, respectively.
Gels are three dimensional, networked polymeric structures, formed by crosslinking reactions which occur due to thermal and/or thermo-oxidative degradation. Gel formation is the most severe form of thermally induced degradation, involving the reaction of products produced during earlier stages of the degradation process. It has been postulated that gels are formed via free radical reactions.
Conditions particularly conducive to thermally induced degradation and subsequent gel formation may be present in the polycondensation reactors which produce PET resin on a commercial scale. As discussed above, the polycondensation process involves reacting the PET prepolymer at an elevated temperature, i.e. 275.degree.-300.degree. C. In addition to elevated temperatures conducive to gel formation, massive commercial reactors may also contain "dead spots" due to design imperfections. These "dead spots" are areas near the reactor walls which receive heat, but little or no agitation. PET prepolymer may remain trapped in these "dead spots" for days or even months prior to breaking loose from the reactor wall and passing through the remaining polymer process, giving rise to gels in the final product. Process upsets such as vacuum line air leaks further feed the thermo-oxidative degradation process. In addition, metal catalysts such as those commonly used to increase PET production can decrease the thermal stability of the polymer, thereby forming gels.
The elimination of thermally induced degradation leading to gels in PET resin is important because the presence of gels interferes with the further processing of the polymer chip, as well as the acceptability of the polymer for certain applications. Gels give rise to a variety of difficulties during polymer processing, including high plugging rates in process filters, high yarn break rates, and poor runnability in film manufacturing. In particular, softer gels, which pass through the extrusion process, give rise to broken filaments in fiber production, leading to lower yields. Further, the presence of gels in the extruded product may render PET resin used in film unsuitable for use in high magnification applications, such as microfilm.
Accordingly, a need exists for a commercially acceptable PET resin which generates significantly fewer gels during polymer formation.
In addition to the need for fewer gels, PET resin suitable for use in fiber, film, and molding applications must have adequate IV and color.